Here,I have two problem:
1:when I select a value, I can not make it return to empty status (Please select...);
2:the displayField is a i18n resource,how to change the typeName just like renderer method of ColumnModel.

To get the value back to nothing in a combo, you need to either call the reset () function for the combo, or you need to have a record in your data store with a blank value that can be chosen by the user.